Sliding velocity dependence of adhesion in a nanometer-sized contact

The influence of sliding velocity on the adhesion force in a nanometer-sized contact was investigated with a novel atomic force microscope experimental setup that allows measuring adhesion forces while the probe is sliding at continuous and constant velocities. For hydrophobic surfaces, the adhesion forces (mainly van?der?Waals forces) remain constant, whereas for hydrophilic surfaces, adhesion forces (mainly capillary forces) decrease linearly with a logarithmic increase of the sliding velocity. The experimental data are well explained by a model based on a thermally activated growth process of a capillary meniscus.     

原子力显微镜探针耦合变形下的微观扫描力研究

原子力显微镜(AFM)的微探针系统是典型的微机械构件，它在接触扫描过程处于耦合变形状态.采用数值模拟方法探究恒力模式下探针耦合变形对微观扫描力信号、微观形貌信号的影响.研究表明，AFM的恒力模式扫描中，法向扫描力并不是恒定大小，与轴向扫描力存在耦合作用，在粗糙峰峰值增加阶段，二力均增加；在粗糙峰峰值减小阶段，二力均减小；该耦合作用随形貌坡度、针尖长度等增加而加强.微观形貌的测试信号和横向扫描侧向力信号受探针耦合变形影响较小，但侧向力与形貌斜率密切相关，且其极值点与形貌极值点存在位置偏差，这些结果均与原子力 关键词： 原子力显微镜 探针悬臂梁 耦合变形 扫描力     

Simulation of Properties of Images with Atomic Resolution in a Scanning Probe Microscope

A method for simulation of images in a scanning probe microscope (SPM) using simultaneous wavelet transform and median filtering is proposed. The wavelet transform with the fourth-order Daubechies kernel is used. Such a transform makes it possible to select details of different scales in the SPM image and, hence, study fractal properties of surfaces. Simulation is used to show that ultrahigh (atomic) resolution is possible in SPM provided that the size of the contact region in the probe–sample system is significantly greater than atomic size and the lattice atoms are randomly distributed. Contrast inversion in the SPM images in the multiscan mode is interpreted.     

Fluctuation-induced electromagnetic interaction of a moving particle with a plane surface

The most general (nonrelativistic) formulas for the force of attraction to the surface and for the drag of a nonrelativistic atom moving parallel to it, as well as for the lateral and normal forces acting on a moving dipole molecule and on a charged particle (in the case of parallel and perpendicular motion), are derived for the first time in the framework of the fluctuational electromagnetic theory. The dependences of these forces on the velocity, temperature, separation, and dielectric properties of the atom and the surface are derived. The effect of the nondissipative resonance interaction between a moving neutral atom and the field of surface plasmons, as well as the possible emergence of a positive (accelerating) force acting on the atom (nanoprobe), is substantiated theoretically. The role of dynamic fluctuational forces and their possible experimental measurement when using the quartz microbalance technique and an atomic-force microscope (in the dynamic mode), as well as during deceleration of atomic beams in open nanotubes, are considered. The correctness of the obtained results is confirmed by their agreement with most of the available theoretical relations derived by other authors.     

Electrical nanoprobing of semiconducting carbon nanotubes using an atomic force microscope

We use an atomic force microscope (AFM) tip to locally probe the electronic properties of semiconducting carbon nanotube transistors. A gold-coated AFM tip serves as a voltage or current probe in three-probe measurement setup. Using the tip as a movable current probe, we investigate the scaling of the device properties with channel length. Using the tip as a voltage probe, we study the properties of the contacts. We find that Au makes an excellent contact in the p region, with no Schottky barrier. In the n region, large contact resistances were found which dominate the transport properties.     

Investigation of the mechanical and electronic properties of Ag-Au and Co-Au nanocontacts by the method of first-principle molecular dynamics

The atomic and electronic structures of AgAuAg, AuAgAg, and AuCoCo mixed nanocontacts are investigated by the method of first-principle molecular dynamics. The characteristic interatomic distances in contacts depending on their component composition are determined. The interrelationship between the structural and electronic properties of mixed nanocontacts is shown. The formation of stable bonds between the atoms of various elements in the contact chain, which makes it possible to explain the cause of stabilization and an increase in the contact strength at large interatomic distances is found. It is shown that the addition of Co atoms into pure-gold nanocontacts increases their strength (increases the breaking force of the contact), while the addition of Ag atoms leads to an increase in the range of interatomic distances at which the existence of the nanocontact is possible.     

A simplified but intuitive analytical model for intermittent-contact-mode force microscopy based on Hertzian mechanics

The forces acting on the substrate in intermittent-contact-mode (IC mode, tapping mode) atomic force microscopy are not accessible to a direct measurement. For an estimation of these forces, a simple analytical model is developed by considering only the shift of the cantilever resonance frequency caused by Hertzian (contact) forces. Based on the relationship between frequency shift and tip–sample force for large-amplitude frequency-modulation atomic force microscopy, amplitude and phase versus distance curves are calculated for the intermittent contact mode, and the forces on the substrate are calculated. The results show a qualitative agreement with numerical calculations, yielding typical maximal forces of 50–150 nN. When working above the unperturbed resonance, forces are found to be significantly larger than below the resonance.     

Can atomic force microscopy achieve atomic resolution in contact mode?

Atomic force microscopy operating in the contact mode is studied using total-energy pseudopotential calculations. It is shown that, in the case of a diamond tip and a diamond surface, it is possible for a tip terminated by a single atom to sustain forces in excess of 30 nN. It is also shown that imaging at atomic resolution may be limited by blunting of the tip during lateral scanning.     

Scanning probe microscope with interchangeable AFM-FFM and STM heads

Summary A scanning probe microscope operating in air with interchangeable atomic force-friction force (AFM-FFM) and electronic-tunnelling (STM) heads is presented. Our AFM operates in the so-called contact mode and utilizes the optical-lever detection method which allows simultaneous measurement of the topography as well as the lateral force. The set-up also contains an optical microscope to control both the sample and the probe laser spot on the cantilever. The experimental method to change from AFM to STM operation is based on the use of the probe laser beam and the optical microscope. The maximum scanning area is (24×24) μm² and it is well embraced in the optical-microscope visual field. The microscope attains atomic resolution in air in both AFM and STM configuration. Its performance is demonstrated on the surface of different samples. In honour of Prof. Fausto Fumi on the occasion of his retirement from teaching.     

The Colloidal Probe Technique and its Application to Adhesion Force Measurements

Knowledge of the interaction forces between colloidal particles and surfaces is a precondition for understanding the stability of dispersed systems and adhesion phenomena. One of the methods available for direct measurement of surface forces is the atomic force microscope (AFM). Based on this method the so called “colloidal probe technique” was developed more than 10 years ago. Using a micron‐sized particle glued to the end of an AFM cantilever as the force sensor, this technique is predestined for the study of colloidal interactions. In this review we describe the colloidal probe technique and give an overview of its application in the field of adhesion forces.     

Contact dynamics of tapping mode atomic force microscopy

A comprehensive model on the dynamics of a tilted tapping mode atomic force microscopy (AFM) is presented, which includes the multimodal analysis, mode coupling mechanisms, adhesion, contact and friction forces induced by the tilting angle. A displacement criterion of contact/impact is proposed to eliminate the assumptions of the previous models such as infinite stiffness of sample or zero impact velocity, which makes the model presented here suitable for more general AFM application scenario, especially for the soft sample case. The AFM tip mass, tip–sample damping, contact forces and intermittent contact can all induce the higher modes participation into the system motion. One degree of freedom or one mode study on the AFM contact dynamics of tapping mode is shown to be inaccurate. The Hertz and Derjaguin–Muller–Toporov models are used for the comparison study of the non-adhesive and adhesive contacts. The intermittent contact and the contact forces are the two major sources of the system nonlinearity. The rich dynamic responses of the system and its sensitivity to the initial conditions are demonstrated by presenting various subharmonic and nonperiodic motions.     

Deformation of the contact area and adhesive friction between a scanning frictional microscope probe and the atomic-flat surface

The determination of the equilibrium atomic structure of a nanotribocontact, formed by a hard probe to be viewed as a paraboloid of revolution and subjected to an external load, with the soft surface modeled by a set of parallel atomic planes is considered. Structural, energy, and load characteristics are calculated. In addition, dissipative static adhesive friction as a function of the normal load and the radius of probe curvature for the diamond-graphite system is derived. A number of approximations of the interatomic potentials is used. It is shown that an allowance for the deformation of the contact area causes the adhesive frictional force in the tensile (negative) load range to decrease. For positive loads in a range of 0–80 nN, the variation of the frictional force (when deformation is taken into account) depends on the radius of the probe curvature and the used approximation of the interaction potential. The dependence of friction on the radius of the probe curvature is close to a direct proportionality. The calculated results are compared with the available experimental data.     

Role of vacancies and adsorbed atoms in the channeling of molecular particles in CNTs

The channeling of atomic and molecular particles in carbon nanotubes is considered in the presence of vacancies on the walls and of adsorbed atoms inside them. It is shown that the significant influence of the indicated disturbance of nanotube structures greatly affects channeling, which makes it possible to use beams of atomic and molecular particles to probe nanotubes.     

Scattering of Attosecond Electromagnetic Field Pulses by Atomic and Molecular Anions Including the Magnetic Field Component

Scattering of an attosecond electromagnetic field pulse by atomic and molecular anions is considered in the sudden perturbation approximation. In the interaction with anions, the effect of the magnetic field of the incident pulse is considered. The inclusion of the magnetic component of the ultrashort pulse leads to new results in the scattering spectra for atomic and molecular anions. Analytic dependences of the partial spectrum on frequency are obtained. It is shown that during scattering, the second harmonic is generated, which makes it possible to determine the type of anions and the spatial orientation of molecular anions.     

High resolution atomic force microscopic imaging of the Si(111)-(7 x 7) surface: contribution of short-range force to the images

Observation of the rest-atom layer of the Si(111)-(7 x 7) surface is performed by atomic force microscopy. By detecting the force due to the single chemical covalent bond formed between the tip and the sample surface, individual atoms on the layer were clearly resolved. Unprecedented high spatial resolution was achieved by setting the detection force at a small value and by reducing background forces due to the long-range interactions with the small oscillation amplitude of the cantilever and sharp probe tip.     

Dynamics of moving interacting atoms in a laser radiation field and optical size resonances

The forces acting on interacting moving atoms exposed to resonant laser radiation are calculated. It is shown that the forces acting on the atoms include the radiation pressure forces as well as the external and internal bias forces. The dependences of the forces on the atomic spacing, polarization, and laser radiation frequency are given. It is found that the internal bias force associated with the interaction of atomic dipoles via the reemitted field may play an important role in the dynamics of dense atomic ensembles in a light field. It is shown that optical size resonances appear in the system of interacting atoms at frequencies differing substantially from transition frequencies in the spectrum of atoms. It is noted that optical size resonances as well as the Doppler frequency shift in the spectrum of interacting atoms play a significant role in the processes of laser-radiation-controlled motion of the atoms.     

Theoretical Study on the Capillary Force between an Atomic Force Microscope Tip and a Nanoparticle

Considering that capillary force is one of the most important forces between nanoparticles and atomic force microscope （AFM） tips in ambient atmosphere, we develop an analytic approach on the capillary force between an AFM tip and a nanoparticle. The results show that the capillary forces are considerably affected by the geometry of the AFM tip, the humidity of the environment, the vertical distance between the AFM tip and the nanoparticle, as well as the contact angles of the meniscus with an AFM tip and a nanoparticle. It is found that the sharper the AFM tip, the smaller the capillary force. The analyses and results are expected to be helpful for the quantitative imaging and manipulating of nanoparticles by AFMs.     

Single atomic contact adhesion and dissipation in dynamic force microscopy

By combining dynamic force microscopy experiments and first-principles calculations, we have studied the adhesion associated with a single atomic contact between a nanoasperity--the tip apex--and a semiconductor surface--the Ge(111)-c(2 x 8). The nanoasperity's termination has been atomically characterized by extensive comparisons of the measured short-range force at specific sites with the chemical forces calculated using many atomic models that vary in structure, composition, and relative orientation with respect to the surface. This thorough characterization has allowed us to explain the dissipation signal observed in atomic-resolution images and force spectroscopic measurements, as well as to identify a dissipation channel and the associated atomic processes.     

On levitation of diamagnetic bodies in a magnetic field

Various methods for calculating the force characteristics of a suspension ensuring levitation of diamagnetic bodies of various origin and shape are considered. The method of calculation is based on the computation of the energy of interaction of a body upon its displacement from the center of suspension for an arbitrary configuration of the suspending field. The method of quasi-homogeneous approximation is considered and compared with the method for calculating the force characteristics based on the general formula for the magnetic energy. The necessary and sufficient conditions for conservative stability of the equilibrium state are formulated, and the stability domain is determined. It is shown that the stability domain depends on the size of the body. Calculations of force characteristics are performed specifically for the suspension of a diamagnetic sphere. The dependences of forces and stiffnesses on the magnetic field strength are obtained, which makes it possible to analyze the stability of confinement of the diamagnetic sphere in the field of the system of circular currents.     

Correlation between elastic anisotropy and magnetic susceptibility anisotropy of sedimentary and metamorphic rock

A correlation has been revealed between the types of acoustic anisotropy and magnetic susceptibility anisotropy for specimens from noncrystalline metamorphic and sedimentary rock has been revealed. This correlation makes it possible to assume that the anisotropy of magnetic and acoustic characteristics has the same origin, namely, the rock texture. The pronounced triaxial acoustic anisotropy for specimens of polycrystalline rock in the absence of external forces makes it possible to judge the fracturing caused by tectonic loads and the degree of this fracturing. The large body of available experimental data allows the expectation that it will be possible to obtain empirical dependences suitable for estimating the intensity of geological processes based on the values of the magnetic susceptibility anisotropy and elasticity tensor anisotropy.